Cytosine deaminase/5-fluorocytosine gene therapy and Apo2L/TRAIL cooperate to kill TRAIL-resistant tumor cells.

The death ligand Apo2L/TRAIL (Apo2 ligand/tumor necrosis factor-related apoptosis-inducing ligand) eradicates many tumor types while sparing most normal tissues. However, some tumors are resistant to TRAIL. We therefore determined if TRAIL cooperates with cytosine deaminase/5-fluorocytosine (CD/5-FC) gene therapy and investigated the mechanisms involved. Transfection of human LAN-5 neuroblastoma cells with CD rendered the cells (LAN-5-CD) sensitive to 5-FC-induced, caspase-dependent apoptosis. Mediated by caspase-3, CD/5-FC and TRAIL cooperated to induce apoptosis in these TRAIL-resistant cells and to cleave X-linked inhibitor of apoptosis protein (XIAP). In established LAN-5-CD tumors growing subcutaneously in mice, intratumorally applied TRAIL did not decrease tumor growth and systemically administered 5-FC only attenuated tumor growth. In contrast, 5-FC together with TRAIL dramatically decreased tumor growth and eradicated a tumor. Assuming sufficient gene transfer of CD in situ, CD/5-FC with TRAIL may be useful for the therapy of tumors resistant to TRAIL.